Variable displacement axial piston pumps typically have a center pivoted swash plate. Some such pumps use trunnion bearings while others use a half shell or cradle support having either sliding or roller bearings. Those pumps have the advantage of simplicity in construction and logic, but also have the disadvantage that as displacement is adjusted from maximum to zero the pistons settle in on their mid-stroke position. This results in the piston cylinders retaining a volume of "trapped" oil therein during operation with the trapped volume being dependent upon the displacement setting of the pump. The trapped volume varies from essentially zero at maximum displacement of the pump to about half the displacement volume of the pump at its zero displacement setting. Oil is slightly compressible under high pressure and a considerable amount of energy is required to compress the oil at high pressures normally achieved by axial piston pumps. The energy is recovered from the oil pumped from the pump, but much of the energy expended in compressing the volume of trapped oil is lost.
Thus, it is desirable to provide a variable displacement axial piston pump in which the trapped volume of oil is held to a minimum therein throughout the full range of the displacement settings of the pump.
The present invention is directed to overcoming one of more of the problems as set forth above.